This commit adds the Purebred.Types.IFC module for information flow
control (IFC) types and functions.  We begin with a trivial taint
mechanism.  A single aspect of the implementation - tryRunProcess -
is updated to use it, for demonstration and review purposes.

Part of: #269

Taint sendmail stderr.  Also re-export some typed-process
ProcessConfig-related types and functions from
Purebred.System.Process.  This is a move to avoid importing
System.Process.Typed anywhere in purebred, except this one module,
ensuring that all the "run process" functions that are available
taint the process output.

Part of: #269

Create tainting version of readProcess and update
Storage.Notmuch.getDatabasePath to use it.

Part of: #269

Re-export 'shell' from Purebred.System.Process so that we can avoid
directly importing System.Process.Typed anywhere (except the
aforementioned module).  We want to avoid using typed-process
directly; instead we want to ensure we use our own variants that
taint the process output.

Part of: #269

Part of: #269